Ground Cover Supplement : GC Supplement - Profit from crop nutrition 2013
12 Soil tests MORE ACCURATE SOIL TESTS SOUGHT The potential for potassium and sulfur deficiency to become prevalent in some Australian cropping systems is increasing due to relatively low application rates of these nutrients and substantial removal in harvested products RECENT REPORTS OF sulfur and potassium deficiencies in Australian crops have exposed how little we know about critical values for these nutrients and has prompted development of more accurate soil tests for these important crop nutrients. Leading the development is University of Adelaide researcher Dr Sean Mason, who says that with limited soil and tissue testing for potassium and sulfur it is difficult to quantify the extent of the deficiency issue across Australia. "The move away from using single superphosphate fertilisers has meant sulfur inputs have declined and it is therefore not surprising that sulfur deficiency has started to emerge, particularly in sensitive crops such as canola," he says. "We also have reports of potassium deficiency from two very different soil types -- on sandy soils in Western Australia and on clay vertosols in Queensland." Dr Mason says there is little information about how well current potassium and sulfur soil tests correlate with crop responses, particularly for areas outside of the western growing region. "But we do know they have only a moderate predictive capability of identifying a sulfur or potassium deficiency," Dr Mason says. NEW TESTS ON THE HORIZON The mobile nature of sulfur in the soil and its capacity to be mineralised and oxidised within cropping systems make it a challenge to reliably predict when a crop will be responsive to this nutrient. "As it is difficult to capture how much sulfur is in the soil at any one time, any new test will need to predict the potential amount of available sulfur through the course of the growing season," Dr Mason says. Potassium is similar to phosphorus in terms of the way it behaves in the soil -- with little capacity to diffuse through most soil types. "We may be able to adapt the recently released diffusive gradient in thin-films (DGT) phosphorus test for potassium and sulfur," Dr Mason says. Developing the new tests will be done as part of the second phase of the More Profit from Crop Nutrition program. "We'll be using the latest technology to develop an accurate soil test for sulfur and potassium, one that quantifies the amount of sulfur and potassium initially available for crop uptake and the amount expected to become available during the early part of the growing season." □ Grdc research code ua00140 More information: Dr Sean Mason, 0422 066 635, email@example.com Soil testing for crop nutrition Fact sheet: www.grdc.com.au/GRDC-FS-Soiltesting colWell-P vs dGt-P Growers now have access to two phosphorus soil tests, DGT-P and Colwell-P. So which test should be used and where? Current knowledge suggests in most cases it does not matter which phosphorus test is used as long as the same test is used consistently across paddocks and years. “If you have a high-calcium soil then the new DGT-P test will be superior to Colwell-P, but on any other soil type both DGT-P or Colwell-P will work well,” says Dr Simon Speirs, national coordinator of the More Profit from Crop Nutrition program. “The main thing is to be consistent – use the same soil test on each paddock over time otherwise you won’t be able to compare the results.” Colwell-P needs to be used in conjunction with a phosphorus buffering index (PBI) value to take account of the amount of phosphorus being tied up or ‘fixed’ within the soil. Dr Speirs also encourages the use of soil labs with Australasian Soil and Plant Analysis Council (ASPAC) certification for these tests. By providing certification on an annual basis for specific soil tests, ASPAC provides users with an indication of a laboratory's level of proficiency. A list of certifed laboratories can be found on the ASPAC website, www.aspac-australasia.com New and more accurate soil tests for potassium and sulfur are being developed.
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